A sputtering method is employed as one of the effective methods of forming a refractory metal silicide thin film used for a gate electrode, source electrode, drain electrode of semiconductor devices such as MOS, LSI devices and the like and for wiring. The sputtering method, which is excellent in mass-productivity and the stability of a formed film, is a method such that argon ions are caused to collide with a disc-shaped refractory metal silicide target and discharge a target constituting metal which is deposited as a thin film on a substrate disposed in confrontation with the target. Consequently, the property of the silicide thin film formed by sputtering greatly depends upon the characteristics of the target.
Recently, as a semiconductor device is highly integrated and miniaturized, it is required that a sputtering target used to form a refractory metal silicide thin film produces a less amount of particles (fine grains). That is, since particles produced from a target during sputtering have a very fine grain size of about 0.1-10 .mu.m, when the particles are mixed into a thin film being deposited, they cause a serious problem that the yield of semiconductor devices is greatly reduced by the occurrence of short circuit between wires of a circuit and insufficient opening of wires. Thus, the reduction of an amount of particles is strongly required.
Since it can become effective means to miniaturize a target structure, that is, to make the size of MSi.sub.2 grains and free Si grains as small as possible in order to reduce an amount of particles produced from a target, there are conventionally proposed various manufacturing methods of miniaturizing the structure.
For example, Japanese Patent Application Laid-Open No. Sho 63(1988)-219580 discloses that a high density target having a fine structure and containing a small amount of oxygen can be obtained in such a manner that a mixed powder obtained by mixing a high purity refractory metal powder with a high purity silicon powder is subjected to a silicide reaction in high vacuum and a semi-sintered body is formed, then the resultant semi-sintered body is charged into a pressure-tight sealing canister without being crushed and the pressure-tight sealing canister is sintered by a hot isostatic press after having evacuated and sealed. In this case, the thus obtained target has a fine structure having the maximum grain size of MSi.sub.2 not greater than 20 .mu.m and the maximum grain size of free Si not greater than 50 .mu.m and containing oxygen not greater than 200 ppm with a density ratio not less than 99%.
Further, Japanese Patent Application Laid-Open No. Hei 2(1990)-47261 discloses that a high density target with a fine structure can be obtained in such a manner that a mixed powder of a high purity refractory metal powder and a high purity silicon powder is subjected to a silicide reaction in high vacuum and a semi-sintered body is formed, then the semi-sintered body is crushed to not greater than 150 .mu.m and further added and mixed with a high purity silicon powder and charged into a pressure-tight sealing canister, then the pressure-tight sealing canister is sintered by a hot isostatic press after having evacuated and sealed. In this case, the thus obtained target has a maximum grain size of MSi. not greater than 20 .mu.m and a density ratio not less than 99% with only free Si existing in a grain boundary.
Recently, as a semiconductor device is highly integrated and miniaturized, a high purity target containing a very small amount of impurities, which deteriorate the characteristics of the semiconductor device, is required as a sputtering target used to form a refractory metal silicide thin film. In particular, it is strongly required to minimize an amount of oxygen in a target because oxygen, which concentrates on the interface between a silicide layer and an under layer and increases a film resistance, delays signals and lowers the reliability of the device.
Since it is effective oxygen reducing means to make deoxidation by heating a semi-sintered body as a material in vacuum and volatilizing oxygen in the form of silicon oxide (SiO or SiO.sub.2), the following manufacturing methods of reducing oxygen are conventionally proposed.
For example, Japanese Patent Application Laid-Open No. Sho 62(1987)-171911 obtains Mo silicide or W silicide each containing a small amount of oxygen in such a manner that a mixed powder obtained by mixing a Mo powder or W powder with a Si powder is heated in vacuum at a temperature less than 800-1300.degree. C. and a Mo silicide powder or W silicide powder is synthesized, then the resultant powder is held in vacuum at 1300-1500.degree. C. to remove oxygen as SiO by excessive Si.
On the other hand, a trial for optimizing the grain size of a material powder and hot pressing conditions from a view point that the condensation of free Si results to an increase of particles produced and the following manufacturing method is proposed.
For example, Japanese Patent Application Laid-Open No. Sho 63(1988)-74967 obtains a target from which condensed silicon is removed in such a manner that a mixed powder obtained by adding a synthesized silicide powder of -100 mesh with a silicon powder of -42 mesh is heated to 1300-1400.degree. C. while applying a preload of 60-170 kg/cm.sup.2, then pressed with a pressing pressure of 200-400 kg/cm.sup.2 and held after being pressed.
Further, Japanese Patent Application Laid-Open No. Sho 64(1989)-39374 obtains a target from which condensed silicon is removed in such a manner that two types of synthesized silicide powders of -100 mesh having a different composition are prepared and a mixed powder adjusted to have an intended composition is hot pressed under the same conditions as above.
There is a problem, however, that when all the amounts of a mixed powder necessary to form a single target is subjected to a silicide synthesis at once in high vacuum in the above conventional manufacturing methods, resulting MSi.sub.2 grains are rapidly grown and coarsened as well as cracks are made to an entire semi-sintered body by a rapidly increased temperature in a silicide reaction because the silicide reaction is an exothermic reaction, and when the semi-sintered body is sintered by pressing in the state as it is, a resultant sintered body cannot be used because the cracks remain.
There is also a problem that since a mixed material powder overflows from a vessel by the rapid increase of temperature in the silicide reaction and a composition is out of an intended composition due to the volatilization of very volatile Si. Thus, when the semi-sintered body is sintered by pressing in the state as it is, a target having a desired composition cannot be obtained.
Further, there is a problem that even if a semi-sintered body is crushed and made to a powder, since hard MSi.sub.2 particles which have been grown once and coarsened remain without being finely crushed, a target having a uniform and fine structure cannot be obtained as well as an amount of contamination caused by impurities is increased by crushing and in particular an amount of oxygen is greatly increased.
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. Sho 62(1987)-171911, when a mixed powder is subjected to a silicide synthesization at 800-1300.degree. C. and further deoxidized by being heated to high temperature so as to reduce impurity oxygen, there is a problem that since the sintering property of a resultant semi-sintered body is excessively improved, the semi-sintered body cannot be sufficiently crushed in a subsequent crushing process and formed to a segregated structure in which MSi.sub.2 and Si are irregularly dispersed, and in particular, when a heating temperature reaches a temperature region exceeding 1400.degree. C., this tendency is made more remarkable.
Although a semi-sintered body is crushed in an atmosphere replaced with Ar (argon gas) to prevent an increase of an oxygen content, it is difficult to completely prevent the contamination by oxygen when the semi-sintered body is crushed. Further, a problem also arises in that when a crushed powder is taken out from a vessel such as a ball mill or the like, the powder surely adsorbs oxygen to increase oxygen contained therein, and as a result a finely crushed powder has an increased surface area and an amount of oxygen adsorbed by the powder is greatly increased.
On the other hand, even if a synthesized powder was hot pressed while applying a preload of 60-170 kg/cm.sup.2 thereto according to the methods of Japanese Patent Application Laid-Open No. Sho 63(1988)-74967 and Japanese Patent Application Laid-Open No. Sho 64(1989)-39374, condensed silicon was disadvantageously produced and a target having a fine and uniform structure could not be obtained.
Further, when a synthesized powder was hot pressed without being applied with a preload, MSi.sub.2 grains obtained by synthesization was grown as well as a composition has an inclined distribution in a target and it was difficult to obtain a target having a fine and uniform structure.
Japanese Patent Application Laid-Open No. Sho 62(1987)-70270 discloses a refractory metal silicide target having a density ratio not less than 97%. Further, Japanese Patent Application Laid-Open No. Sho 62(1987)-230676 discloses a methods of manufacturing a refractory metal silicide target and describes that a target is molded by compacting using a single axis under the conditions of high temperature, high vacuum and high pressing pressure.
However, the above respective prior arts describe only that a target is made by subjecting a material powder for the target to hot pressing and no description is made as to a fine and uniform structure. Thus, these prior arts cannot achieve an object for effectively suppressing particles.
On the other hand, International Patent Application published according to PCT (No. W091/18125 discloses a silicide target having 400.times.10.sup.4 pieces of silicide with a grain size of 0.5-30 .mu.m existing in a cross section of the mixed structure of the target of 1 .mu.mm.sup.2 with the maximum grain size of Si not greater than 30 .mu.m and further a silicide target with the average grain size of silicide of 2-15 .mu.m and the average grain size of Si of 2-10 .mu.m.
Since the manufacturing method described in the prior art is insufficient to obtain a fine uniform target structure, the object to suppress the occurrence of particles cannot be sufficiently achieved.
An object of the present invention is to provide a high density and purity refractory metal silicide target which has a fine mixed structure and a uniform composition as well as contains a less amount. of impurities such as oxygen and the like, a method of manufacturing the target, a refractory metal silicide thin film and a semiconductor device.